Each control cluster consists of absorbing rods suspended from a multi-arm support referred to as a "spider". This support is fixed to the lower end of a vertically operated bar. The absorbing rods must be able to be removed from the core or be introduced into the core, which requires perfect guiding over the entire length of their displacement.
A device for guiding a control cluster, which is in itself known, comprises a tubular casing extending between an upper core plate, which limits the core at the top, and a support plate. Horizontal plates, which make it possible to discharge the primary cooling liquid upwards, are located inside the guide casing in order to ensure discontinuous guiding of the rods. At the lower level, the casing contains a set of guide sleeves provided with slides which ensure the continuous guiding of the rods. Each guide sleeve possesses a longitudinal slot allowing the radial elements of the "spider" to pass through. At a height corresponding to the continuous guiding, the casing is provided with openings for discharging and diverting the primary cooling liquid. The guide casing receives the rising flow of liquid, which leaves the fuel arrays vertically, at the upper end of the core, and is then diverted horizontally towards the lateral outlets in the vessel. The casing is therefore located in a highly turbulent zone.
At the level of the continuous guiding, there is a pressure gradient in a horizontal plane. Because each guide sleeve is open, this pressure gradient tends to press the absorbing rod against the lips of the slot, causing wear by friction. To limit this phenomenon, pressure-equilibrating openings are provided in the faces of the guide sleeves, but this runs the risk of influencing the mechanical strength.